The invention relates to a transmission means with a transmission signal source and a transmission antenna for the radiation of radio signals of a determined transmission frequency according to a predetermined time protocol.
In radio systems, such as e.g. the GSM mobile radiotelephone system, transmission means are used for the radiation of radio signals as electromagnetic waves that are recorded and evaluated by receiving means. There are thus no line-bound connections between the transmission means and the receiving means. In the transmission means, the transmission signals are produced in a transmission signal source; these transmission signals are thereby at radio frequency. Via cable connections and various further means such as preamplifiers, etc., the transmission signals are supplied to a transmission antenna, which finally radiates the radio signals.
A transmission antenna thereby comprises a particular radiation characteristic. Under real conditions of use for radio systems, the radio signals are exposed to a wide variety of disturbances, and reach the receiver means via very different propagation paths. Besides a direct propagation path, the radio signals can also be reflected or bent at obstacles such as mountains, trees, buildings, etc. In the receiver means, the radio signals from the various propagation paths are superposed. This leads to extinguishing effects, which sometimes have a strong adverse effect on the reception of the radio signals; on this, see J. D. Parsons, "The Mobile Radio Propagation Channel," Pentech Press Publishers, London, 1992, pp. 108-113.
In order to circumvent these extinguishing effects, also called fading effects, various methods are known. With several receiving antennas in the direction of reception, it is for example possible to reduce the extinguishing effect by means of antenna diversity. In addition, from the GSM mobile radiotelephone system it is known to effect an improvement of the reception conditions via frequency hopping, i.e. to change the transmission frequency for the transmission signals (M. Mouly, M. B. Pautet, "The GSM System for Mobile Communications," 1992, i.a. pp. 218-223). However, the latter method has the disadvantage that it requires an expensive implementation. Also, it cannot be used if only one carrier frequency is available in the GSM mobile radiotelephone system, as is the case above all in simple and economical base stations.
From Prior art references EP-A-0 479 744, WO-A-95/32558, and GB-A-2 221 820, communication systems are known in which switching takes place between different antennas during a connection. These antennas can also comprise a different polarizations. In addition, in International reference WO-A-95/32558, the changeover is synchronized with a time protocol of the transmission.
In addition, the problem of strong fluctuations in signal level at the receiver can be compensated by increasing the transmission power. However, this has the disadvantage that increased disturbances must be reckoned with on adjacent radio paths.